Awad Sultan

1D and 3D resistivity inversions for geotechnical investigation

The resistivity method is frequently used in the investigation of the shallow parts of the earth. Interpretation of such data is usually done assuming a layered earth. However, a more complete imaging can be obtained if 3D models are used. Thirty-five vertical electrical soundings (VES) were carried out in a regular mesh at the northwestern part of Greater Cairo in order to characterize different geological units and to study their quality for building foundations. Models obtained from 1D inversion of each VES, together with borehole information, were used for construction of eight geoelectrical sections which exhibit the main geoelectrical characteristics of the geological units present in the area. The 3D inversion of the data indicated a complex subsurface electrical resistivity distribution conditioned by lithology, water content and tectonic structures. The results indicate that the subsurface consists of different geologic units such as gravel and sand, sand, clay and limestone. The main results are related to the characterization of the clay formations in the north of the survey area, which is revealed by low-resistivity values ( 600 Ω m) depicted in the central part of the study zone.

A study of the groundwater seepage at Hibis Temple using geoelectrical data, Kharga Oasis, Egypt

Geoelectric techniques have been used to detect and define the subsurface stratigraphy and structures around Hibis Temple in Kharga Oasis, Egypt. We used 2D and 3D inversion approaches to interpret the data set obtained from 20 dipole-dipole resistivity profiles with electrode spacings of 3 and 5 m and lengths between 39 and 85 m. Five vertical electrical soundings, with a maximum array length of 200 m, along a profile crossing the study area were also carried out. A preliminary quantitative interpretation of the vertical electrical sounding curves was achieved using two-layer standard curves and generalized Cagniard graphs. The final models were obtained by 1D inversion using the results of the manual interpretation as initial models. Model results were used to construct a geoelectric cross-section that correlated very well with the stratigraphic units. Five geoelectric units were identified: the first (the uppermost) is a high-resistivity layer consisting of fill deposits (rubble); the second is a muddy clay with moderate resistivity values; the third is also a muddy clay but with decreased resistivity due to the increase in salt content originated by the evaporation of the groundwater seepage; the fourth unit, at a depth of 7–13 m, is a muddy clay saturated with water seepage from the agricultural areas surrounding the temple; the final unit is a more resistive layer corresponding to dry muddy clay. The differences in the groundwater level, and its salt content, correlated with the irrigation activities around the temple. We concluded that the high corrosion potential of the seepage water might be connected with its salt content.

Application of the resistivity/gravity joint inversion technique for Nubian sandstone aquifer assessment on the area located at the central part of Sinai, Egypt

Eleven deep vertical electrical soundings of AB/2 spacing ranging from 5 to 3000 m were carried out to investigate the upper part of the Nubian groundwater aquifer at the central part of Sinai, Egypt. These soundings have been jointly inverted using the SA algorithm with 160 gravity stations measured in the study area, assuming that density and resistivity contrast are represented by coincident interfaces. One hundred and sixty magnetic stations were executed at the same locations as gravity measurements to estimate the depth of basement rocks. The results of the joint interpretation indicated that the depth of the groundwater aquifer ranges from 500 to 800 m with resistivity values ranging from 6 to 562 Ω m, suggesting that the fresh water is of good quality towards the northern part of the area. The top of the basement, which is mainly defined by gravity and magnetic data, lies at a depth ranging from 830 to 2788 m. The results also show that the aquifer configuration is controlled by different regional faults in the NNW–SSE direction.

Joint inversion interpretation for gravity and resistivity data: a case study at New Heliopolis City, Cairo, Egypt

Gravity and resistivity data were used in the present study for groundwater exploration using a joint inversion process. One hundred fifty three gravity stations and thirty two vertical electrical soundings were used to determine the thickness and resistivity distribution of the different geological units in the study area. Then, the gravity and resistivity data were utilized concurrently to identify the structural elements, stratigraphic units and groundwater potential in the study area. The results of the joint inverse interpretation have strengthened the analysis and consistency of the outcome. The results revealed the presence of four layers. The first is the Quaternary deposits of thickness ranging from 0.5–2 m; the second layer is the Hommath Formation of Middle Miocene age deposits, which have resistivity ranging from 2–3238 Ωm and thicknesses ranging from 5–40 m. The third layer is the Gabal Ahmar Formation (Oligocene age), which has resistivity ranging from 5–1148 Ωm and a thickness ranging from 10–342 m. The fourth layer is limestone of the Upper Eocene age that exhibits high resistivity values ranging from 80–683 Ωm, depths ranging from 55–350 m and bulk density 2600 kg/m 3 . The results also indicate that the area is dissected by different fault elements trending N-S, NE-SW, NE-SW and E-W.